1. Field of the Invention
The present invention relates to a forgery prevention technique.
2. Description of the Related Art
Authentication articles such as cash cards, credit cards and passports and securities such as gift certificates and stock certificates are desired to be difficult of forgery. For this reason, a label which is difficult of forgery or imitation and which makes it easy to distinguish a genuine article from a forged article or an imitated article has conventionally been attached to such an article in order to suppress the forgery.
Further, in recent years, circulation of forged articles is regarded as a problem also for articles other than the authentication articles and securities. For this reason, opportunities have been increasing to apply the forgery prevention technique mentioned for the authentication articles and the securities to such articles.
Patent document 1 describes a display in which multiple pixels are arranged. In this display, each pixel includes a relief-type diffraction grating in which grooves are arranged.
This display displays an image by utilizing diffracted light, and hence it is impossible to forge the display using the printing technique or electrophotographic technique. Accordingly, if this display is attached to an article as a label for authentication, seeing the image displayed on the label makes it possible to confirm that the article is genuine. Therefore, an article to which this label is attached is hardly forged as compared with an article to which this label is not attached.
The above-mentioned relief-type diffraction grating, however, can be formed with comparative ease if a device such as a laser is available. Further, in the above display, although a change in the display image is caused by changing an incident angle of the illumination light, an observation angle or an orientation of the display, the change is not so rich in variety. Therefore, with the development of the technology, the forgery prevention effect of this display is becoming lower. Incidentally, difficulty of forgery or imitation, or ease in distinction of a genuine article from a forged or imitated article is called here a forgery prevention effect.    Patent document 1: Jpn. Pat. Appln. KOKAI Publication No. 2-72320